The present invention concerns a spacecraft payload architecture.
The design and specification of "space segment" telecommunication payloads are evolving in line with operator requirements reflected by the "explosion" in data communications, the development of direct and semi-direct TV broadcast systems, communications with air, land and sea mobiles, etc.
Unlike systems of previous generations, new systems must provide for a number of features directed to improving the management of the satellite's "onboard" resources such as
--multi-beam operation; reconfigurable coverage zones, flexibility of addressing the RF (Radio Frequency) output power to different beams, EIRP (Equivalent Isotropic Radiated Power) reconfigurability.
These features go hand in hand with expected improvements in transmission systems, such as increased transmit EIRP, increasingly high receive G/T factor of merit, and radiated beam quality including: beam shaping: (severe template,) isolation areas, and high directivity.
Specifying the optimum payload for a given mission requires a rigorous review of all potential antenna solutions to develop the best possible system.
Conventional transparent repeater architectures do not offer satisfactory response in the context of multibeam operation with instantaneous power reconfigurability, a concept which has attracted considerable research effort over the last decade.
The natural expression of total instantaneous power reconfigurability is the phased array. The antenna system representing the best response to this problem is essentially the direct radiating array using strictly the same amplitude law to generate any spot where the only control which is then applied is phase control. In a system of this kind, the amplifier rating is immediately apparent and their operation is optimized because there is no differential demand between the different spots, i.e., the amplitude laws are identical. The drawback of this type of approach is its intrinsic complexity if significant directivity is required. The maximum size of the basic components is determined by the array lobe criterion. A large number of unit sources involves a large number of control points and therefore as many amplifiers as there are channels at the beam forming level.
Overall payload budgets often rule out direct radiating arrays when the aperture required is several tens of wavelengths. The result is all the more marked for apertures of more than one hundred times the wavelengths which are routinely used in telecommunication applications employing very high gain. It is therefore beneficial to obtain the gain parameter using a wide aperture where a reflector (or lens) is illuminated by a primary source whose complexity depends on the mission.
Here again there are two feasible concepts, distinguished by whether or not they use a Fourier transform.
The first concept is to dispose the feeds in the focal plane of a focussing system. In this case the beams are generated by carefully managing the amplitude and the phase of each feed. A concept of this kind is described in the article "A versatile array reflector antenna: part A - Reception" by R. Lenormand, E. El Shirbini, J. Neron. J. P. Marre, B. Vidal Saint Andre, R. Coirault and E. Rammos (SIO/IEEE/002-88). This concept has a number of inherent advantages. It offers very high synthesis efficiency and requires a minimal number of feeds (this is characteristic of focused systems).
The use of a focusing optical system offers a number of advantages in terms of antenna performance (synthesis quality, high yield, minimal number of feeds with repercussions on the rating, overall dimensions, mass and power consumption of active and control systems). However, the optical system is in a plane where reconfigurability is poor which introduces a major problem with amplifier ratings where an equal-amplitude condition characteristic of phased array operation ensuring good reconfigurability is required in the amplifier plane. This is achieved, broadly speaking, by applying a second Fourier transform to change from a distribution characteristic of a diffraction spot to an equal-amplitude distribution.
The reconfigurability requirement and the rating of the payload therefore impose the use of a Butler matrix or a generalized coupler which must be included in all balances (mass/overall dimensions/dissipation/losses, etc).
A second concept starts from the idea that two same-focus reflectors are equivalent to a double spatial Fourier transform. This approach is behind the implementation of imaging arrays as described in "Limited electronic scanning with an offset-feed near-field gregorian system" by W. D. Fitzgerald (Technical report 486; Sep. 24, 1971, Massachusetts Institute of Technology Lincoln laboratory).
A structure of this kind is complex for use on a satellite. One of the two reflectors is dispensed with and a non-focusing surface is employed for the main reflector which illuminates a primary array as uniformly as possible. In a device of this kind, theory indicates that only the phase has to be controlled to generate spots in the required areas; all of the radiating elements are energized under equal-amplitude conditions. This mode of operation introduces certain radio frequency design constraints such as an uprated reflector by 25 to 40% as compared to a parabolic reflector), and a large array because it is far away from the "focal plane" on its upstream or downstream side.
From the antenna point of view, the second concept raises two problems. The first problems relates to average synthesis quality because only the phase is controlled and reduced efficiency (35 to 45%) because of the radiation lost in the lobes of the array. The second problem relates to large number and uprating of control elements; this represents a penalty with respect to the mass performance.
This solution is attractive in terms of system budgets because its reconfigurability allows poor antenna performance to be compensated by efficient rating of the amplifiers. The evaluation of the continuous power to produce a given EIRP is the concept which is of benefit in budget terms.